Fin Deformation Modulation

ABSTRACT

A method includes forming a plurality of trenches extending from a top surface of a semiconductor substrate into the semiconductor substrate, with semiconductor strips formed between the plurality of trenches. The plurality of trenches includes a first trench and second trench wider than the first trench. A first dielectric material is filled in the plurality of trenches, wherein the first trench is substantially fully filled, and the second trench is filled partially. A second dielectric material is formed over the first dielectric material. The second dielectric material fills an upper portion of the second trench, and has a shrinkage rate different from the first shrinkage rate of the first dielectric material. A planarization is performed to remove excess second dielectric material. The remaining portions of the first dielectric material and the second dielectric material form a first and a second STI region in the first and the second trenches, respectively.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation application of U.S. patentapplication Ser. No. 14/504,149, entitled “Fin Deformation Modulation,”filed Oct. 1, 2014 which application is a divisional application of U.S.patent application Ser. No. 13/769,783, entitled “Fin DeformationModulation,” filed on Feb. 18, 2013, now U.S. Pat. No. 8,895,446 B2,which applications are incorporated herein by reference.

BACKGROUND

With the increasing down-scaling of integrated circuits and theincreasingly demanding requirements to the speed of integrated circuits,transistors need to have higher drive currents with increasingly smallerdimensions. Fin Field-Effect Transistors (FinFET) were thus developed.The FinFETs include vertical semiconductor fins above a substrate. Thesemiconductor fins are used to form source and drain regions, andchannel regions between the source and drain regions. Shallow TrenchIsolation (STI) regions are formed to define the semiconductor fins. TheFinFETs also include gate stacks, which are formed on the sidewalls andthe top surfaces of the semiconductor fins.

Since the aspect ratio of the gaps between the fins becomes increasinglygreater, in the gap filling for forming the STI regions, materials withhigh shrinkage rates are often used. The high-shrinkage-rate materialsshrink significantly when annealed. This causes significant stressesapplied on the fins, and hence the fins may have deformation andcracking.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 11 are cross-sectional views of intermediate stages inthe manufacturing of semiconductor fins and FinFETs in accordance withsome exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

Shallow Trench Isolation (STI) regions, Fin Field-Effect Transistors(FinFETs), and the methods of forming the same are provided. Theintermediate stages in the formation of the STI regions and the FinFETsare illustrated in accordance with exemplary embodiments. The variationsof the embodiment are discussed. Throughout the various views andillustrative embodiments, like reference numbers are used to designatelike elements.

Referring to FIG. 1, semiconductor substrate 20, which is a part ofsemiconductor wafer 100, is provided. In some embodiments, semiconductorsubstrate 20 includes crystalline silicon. Other commonly usedmaterials, such as carbon, germanium, gallium, boron, arsenic, nitrogen,indium, phosphorus, and/or the like, may also be included insemiconductor substrate 20. Semiconductor substrate 20 may be a bulksubstrate or a Semiconductor-On-Insulator (SOI) substrate.

Pad layer 22 and mask layer 24 may be formed on semiconductor substrate20. Pad layer 22 may be a thin film comprising silicon oxide formed, forexample, using a thermal oxidation process. Pad layer 22 may act as anadhesion layer between semiconductor substrate 20 and mask layer 24. Padlayer 22 may also act as an etch stop layer for etching mask layer 24.In some embodiments, mask layer 24 is formed of silicon nitride, forexample, using Low-Pressure Chemical Vapor Deposition (LPCVD). In otherembodiments, mask layer 24 is formed by thermal nitridation of silicon,Plasma Enhanced Chemical Vapor Deposition (PECVD), or plasma anodicnitridation. Mask layer 24 is used as a hard mask during subsequentphotolithography processes. Photo resist 26 is formed on mask layer 24and is then patterned.

Referring to FIG. 2, mask layer 24 and pad layer 22 are etched throughphoto resist 26, exposing underlying semiconductor substrate 20. Theexposed semiconductor substrate 20 is then etched, forming trenches 32(including 32A and 32B). The portions of semiconductor substrate 20between neighboring trenches 32 form semiconductor strips 30 (including30A and 30B). Trenches 32 may be strips (when viewed in the top view ofwafer 100) that are parallel to each other. After the etching ofsemiconductor substrate 20, photo resist 26 (FIG. 1) is removed. Next, acleaning step may be performed to remove a native oxide of semiconductorsubstrate 20. The cleaning may be performed using diluted hydrofluoric(HF) acid, for example.

Trenches 32 include trenches 32A, and 32B that have lateral sizesdifferent from that of trenches 32A. Some semiconductor strips 30 may beclosely located, so that they may be used to form the semiconductor finsof a same FinFET. For example, semiconductor strips 30A are closelylocated from each other, and semiconductor strips 30B are closelylocated from each other. Throughout the description, semiconductorstrips 30A are in combination referred to as strip group 31A, andsemiconductor strips 30B are in combination referred to as strip group31B. The inner spacing S1 (the lateral size of trenches 32A) betweenstrips 30A may be smaller than about 30 nm, or further smaller thanabout 20 nm in accordance with some embodiments. It is appreciated,however, that the values recited throughout the description are merelyexamples, and may be changed to different values. The inter spacing S2(the lateral size of trenches 32A) between strip groups 31A and 31B maybe greater than about 80 nm, or further greater than about 300 nm inaccordance with some embodiments. Ratio S2/S1 may be greater than about10 in accordance with some embodiments.

In accordance with some embodiments, liner oxide 34 is formed intrenches 32 and on the sidewalls of semiconductor strips 30, as is shownin FIG. 3. Liner oxide 34 may be a conformal layer whose horizontalportions and vertical portions have thicknesses close to each other.Liner oxide 34 may be a thermal oxide (such as silicon dioxide) having athickness between about 10 Å and about 40 Å. In some embodiments, lineroxide 34 is formed by oxidizing wafer 100 in an oxygen-containingenvironment, for example, through Local Oxidation of Silicon (LOCOS),wherein oxygen (O₂) may be included as the process gas. In otherembodiments, liner oxide 34 is formed using In-Situ Steam Generation(ISSG), with water steam or a combined gas of hydrogen (H₂) and oxygen(O₂) used to oxidize semiconductor strips 30, The ISSG oxidation may beperformed at an elevated temperature. In yet other embodiments, lineroxide 34 may be formed using a deposition technique such as SelectiveArea Chemical Vapor Deposition (SACVD). The formation of liner oxide 34may result in the rounding of the corners of trenches 32, which reducesthe electrical fields of the resulting FinFETs, and hence improves theperformance of the resulting integrated circuit. In alternativeembodiments, the formation of liner oxide 34 is skipped.

FIG. 4 illustrates the formation of dielectric region 36. Dielectricregion 36 substantially fully fills trenches 32A (FIG. 3). On the otherhand, the bottom portions of trenches 32B are filled with dielectricregion 36, and the top portions of trenches 32B remain unfilled. Thefilling methods may be selected from spin-on, Flowable Chemical VaporDeposition (FCVD), and the like. Dielectric region 36 may includehighly-flowable materials, which tend to fill narrow trenches 32A to ahigher level than wide trenches 32B. In some embodiments, the topsurfaces of the portions of dielectric regions 36 in trenches 32A aresubstantially level with, or higher than, the top surfaces ofsemiconductor strips 30. Height H1 of the portions of dielectricmaterial 36 filling trenches 32A may also be greater than about 70percent, or greater than about 90 percent, height H3 of semiconductorstrips 30. Height H2 of the portions of dielectric material 36 intrenches 32B, on the other hand, may also be smaller than about 50percent, or smaller than about 30 percent, height H3 of semiconductorstrips 30. Height H1 may be the height of the lowest point of theportion of dielectric region 36 that fills the respective trench 32A,and Height H2 may be the height of the lowest point of the portion ofdielectric region 36 that fills the respective trench 32B.

In some embodiments, dielectric region 36 comprises spin-on glass, whichmay include Si—O—N—H. In alternative embodiments, dielectric region 36comprises flowable oxide, which may include Si—O—N—H, Si—C—O—N—H, or thelike. Highly-flowable materials tend to (although not necessarily) havea high shrinkage rate. Hence, dielectric region 36 may have a highshrinkage rate when cured, annealed, and/or solidified. In someembodiments, dielectric region 36 has a shrinkage rate greater thanabout 10 percent, or between about 10 percent and about 30 percent. Inother embodiments, dielectric material 36 has a small shrinkage rate,for example, smaller than about 10 percent, or smaller than about 5percent, when cured, annealed, and/or solidified.

Referring to FIG. 5, an anneal step (represented by arrows 37) isperformed on wafer 100. Dielectric material 36 is solidified as a resultof the anneal. In alternative embodiments, dielectric material 36 issolidified by a curing process separate from the anneal. In someembodiments, the anneal is performed at a temperature between about 500°C. and about 1,200° C., although different temperatures may be used. Theanneal step may be performed for a period of time between about 30minutes and about 120 minutes, for example. As a result of the annealstep, dielectric material 36 may shrink, and the spacing betweensemiconductor strips 30 may be reduced to S3, which may be smaller thanspacing S1 (FIG. 2) by about 3 percent to about 6 percent, for example.FIG. 5 schematically illustrates that due to the shrinkage of dielectricmaterial 36, semiconductor strips 30 in the same strip group 31A (and31B) are slightly bended toward the center of the respective stripgroup. Since the wide trenches 32B are partially filled, the bending ofsemiconductor strips 30 may be significantly less severe than iftrenches 32B are fully filled. Furthermore, since wide trenches 32B arenot filly filled, the pulling force caused by the shrinkage rate ofdielectric regions 36 in trenches 32B is much smaller. Also,semiconductor strips 30 in the same strip group 31A and 31B may bebended inwardly toward the center of the respective strip group, ratherthan bended outwardly.

In some embodiments, depending on what material is comprised indielectric regions 36 before the anneal step, and further depending onthe process conditions of the anneal step, after the anneal, dielectricregions 36 may comprise Si, N, O, and H atoms.

Next, referring to FIG. 6, the remaining portions of trenches 32 arefilled with a dielectric material to form dielectric region 38. The topsurface of dielectric region 38 is higher than the top surface of masklayer 24. Dielectric region 38 may be formed of a material that hasshrinkage rate lower than the shrinkage rate of dielectric region 36. Insome exemplary embodiments, the shrinkage rate of dielectric region 38,when annealed and/or solidified, may be between about 1 percent andabout 5 percent. Dielectric region 38 may be non-flowable at the time itis formed (before any annealing or curing), although it may also beflowable in accordance with alternative embodiments. Dielectric region38 may include silicon oxide, and other dielectric materials, such asSiN, SiC, or the like, may also be used. In some embodiments, dielectricregion 38 is formed using High Aspect-Ratio Process (HARP), High-DensityPlasma CVD (HDPCVD), or the like. In the deposition of dielectric region38, the respective process gases may include tetraethylorthosilicate(TEOS) and O₃ (ozone). Dielectric regions 36 and 38, after beingannealed, may be formed of a same material or different materials.

A planarization such as Chemical Mechanical Polish (CMP) is thenperformed, as shown in FIG. 7, and hence STI regions 40 are formed. STIregions 40 comprise the remaining portions of liner oxide 34, dielectriclayer 36, and dielectric region 38. After the CMP, dielectric layer 36and dielectric region 38 may include a plurality of discrete portions,which are referred to hereinafter as dielectric regions 36 anddielectric regions 38, respectively. Mask layer 24 is used as the CMPstop layer, and hence the top surface of mask layer 24 is substantiallylevel with the top surface of dielectric regions 38 and the top surfacesof dielectric regions 36. Furthermore, the discrete portions of lineroxide layer 34 are referred to as liner oxide layers 34 hereinafter.

FIG. 8 illustrates the annealing of wafer 100, wherein the annealing isrepresented by arrows 44. In some embodiments, the annealing includes awet anneal step, which is performed using In-Situ Steam Generation(ISSG), in which the steam of water is generated, and is driven throughpad oxide layer 22, mask layer 24, and STI regions 40 to reachsemiconductor strips 30. The anneal step may be performed attemperatures between about 800° C. and about 1,050° C. The duration ofthe ISSG may be between about 1 minute and about 20 minutes. Inalternative embodiments, the oxidation may be performed using a dryanneal method, wherein the process gas my include an oxygen-containinggas such as O₂, H₂, N₂, or the like, and the temperature may be betweenabout 200° C., and about 700° C. The duration of the dry anneal may bebetween about 30 minutes and about 120 minutes. In yet alternativeembodiments, the anneal step includes the wet anneal step followed bythe dry anneal step.

As a result of the anneal, the top portions and sidewall portions ofsemiconductor strips 30 are oxidized. The resulting oxides are not shownsince they may comprise the same oxide as liner oxide layers 34. Thevolume of the resulting oxide is greater than the volume of the portionsof semiconductor strips 30 that are oxidized. Accordingly, the totalvolume of semiconductor strips 30 and the resulting oxides is expandedover the volume of the semiconductor strips 30 before the oxidation. Asa result, the shrinkage of dielectric material 36, as shown in FIG. 5,and the shrinkage of dielectric regions 38 in the anneal, are at leastpartially compensated for. The bended semiconductor strips 30 may thusbe straightened, as shown in FIG. 8. In accordance with someembodiments, the process conditions of the annealing, such as theannealing time, the temperature of wafer 100, etc., may be adjusted, sothat after the annealing in FIG. 9, the expansion of materials caused bythe annealing substantially compensates for the shrinkage of dielectricregions 36 and 38, and hence semiconductor strips 30 may have verticalprofiles. In addition, in the annealing, the quality of dielectricregions 36 and 38 is improved, and the difference between dielectricregions 36 and 38 may be reduced, and hence STI regions 40 may appear ashomogeneous regions.

FIG. 9 illustrates the removal of mask layer 24. Mask layer 24, ifformed of silicon nitride, may be removed by a wet process using hotH₃PO₄. Next, the structure shown in FIG. 9 is used to form semiconductorfins through the recessing of STI regions 40. Pad layer 22 is alsoremoved. STI regions 40 are recessed by an etching step, as shown inFIG. 10. Referring to FIG. 10, the portions of semiconductor strips 30protruding over the top surfaces of the remaining STI regions 40 becomesemiconductor fins 42. The recessing of STI regions 40 may be performedusing a dry etch process or a wet etch process. In some embodiments, therecessing of STI regions 40 is performed using a dry etch method, inwhich the process gases including NH₃ and HF are used. In alternativeembodiments, the recessing of STI regions 40 is performed using a wetetch method, in which the etchant solution includes NF₃ and HF. In yetother embodiments, the recessing of STI regions 40 is performed using adilution HF solution, which may have an HF concentration lower thanabout 1 percent.

FIG. 11 illustrates FinFETs 52 formed from the structures shown in FIG.10. In accordance with some exemplary embodiments, gate dielectrics 48are formed to cover the top surfaces and sidewalls of fins 42. Gatedielectrics 48 may be formed through a thermal oxidation, and hence mayinclude thermal silicon oxide. Alternatively, gate dielectrics 48 may beformed through a deposition step, and may comprise high-k dielectricmaterials. Gate electrodes 50 are then formed on gate dielectrics 48. Insome embodiments, each of gate electrodes 50 covers fins 42 that belongto one of the strip groups 31A and 31B, and each of the resultingFinFETs 52 comprises more than one fin 42. The remaining components ofFinFETs 52, including source and drain regions and source and drainsilicides (not shown), are then formed. The formation processes of thesecomponents are known in the art, and hence are not repeated herein. Gatedielectrics 48 and gate electrodes 50 may be formed using a gate-firstapproach or a gate last approach. The details of the gate-first approachor a gate last approach are not described herein.

As shown in FIG. 11, there are two types of STI regions 40. The STIregions 40A that are underlying gate electrode 50 include dielectricregions 36, and are free from dielectric regions 38. The STI regions40B, which are wider than STI regions 40A, and may be the STI regions40B separating FinFETs, may include both dielectric regions 36 and 38.In STI regions 40B, dielectric regions 36 and 38 may be formed of thesame dielectric material, or may be formed of different dielectricmaterials. In one of STI regions 40B, dielectric region 38 is encircledby dielectric region 36, and is further over a bottom portion ofdielectric region 36. Depending on the materials, dielectrics regions 36and 38 may, or may not, have distinguishable interfaces.

In the embodiments of the present disclosure, by applying a firstdielectric material to fill narrow trenches, while leaving some portionsof wide trenches unfilled, the shrinkage of the first dielectricmaterial has a smaller effect to bend the semiconductor fins. Theremaining portions of the trenches may then be filled using alow-shrinkage-rate material. As a result, substantially un-bendedsemiconductor fins may be formed.

In accordance with some embodiments, a method includes forming aplurality of trenches extending from a top surface of a semiconductorsubstrate into the semiconductor substrate, with semiconductor stripsformed between the plurality of trenches. The plurality of trenchesincludes a first trench and second trench wider than the first trench. Afirst dielectric material is filled in the plurality of trenches,wherein the first trench is substantially fully filled, and the secondtrench is filled partially. A second dielectric material is formed overthe first dielectric material. The second dielectric material fills anupper portion of the second trench, and has a shrinkage rate differentfrom the first shrinkage rate of the first dielectric material. Aplanarization is performed to remove excess portions of the seconddielectric material over the semiconductor substrate, wherein remainingportions of the first dielectric material and the second dielectricmaterial form a first and a second STI region in the first and thesecond trenches, respectively.

In accordance with other embodiments, a method includes forming aplurality of trenches extending from a top surface of a semiconductorsubstrate into the semiconductor substrate, with semiconductor stripsformed between the plurality of trenches. The plurality of trenchesincludes a first trench and second trench wider than the first trench. Afirst dielectric material is filled in the plurality of trenches,wherein the first trench is substantially fully filled, and the secondtrench is filled partially. An anneal is performed on the firstdielectric material. After the first anneal, a second dielectricmaterial is formed over the first dielectric material, wherein thesecond dielectric material fully fills the second trench. Aplanarization to remove excess portions of the second dielectricmaterial over the semiconductor substrate. The remaining portions of thefirst dielectric material and the second dielectric material formShallow STI regions.

In accordance with yet other embodiments, an integrated circuitstructure includes a semiconductor substrate, a first and a secondsemiconductor strip over the semiconductor substrate, and a first STIregion between and contacting the first and the second semiconductorstrips. The first STI region includes a first dielectric region. Asecond STI region is over the semiconductor substrate. The second STIregion includes a second dielectric region, and a third dielectricregion encircled by the second dielectric region. The third dielectricregion is further over a bottom portion of the second dielectric region.The first dielectric region and the second dielectric region are formedof a same dielectric material. The first STI region is free fromdielectric regions formed of a same material as the third dielectricregion.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. An integrated circuit structure comprising: asemiconductor substrate; a first Shallow Trench Isolation (STI) regionextending into the semiconductor substrate, wherein the first STI regioncomprises a first dielectric region; a second STI region extending intothe semiconductor substrate, wherein the second STI region comprises: asecond dielectric region, wherein the first dielectric region and thesecond dielectric region are formed of a same dielectric material; and athird dielectric region over a bottom portion of the second dielectricregion; and a gate dielectric over and in contact with the firstdielectric region.
 2. The integrated circuit structure of claim 1,wherein the first dielectric region, the second dielectric region, andthe third dielectric region have top surfaces substantially planar witheach other, and no additional dielectric is between the first dielectricregion and the gate dielectric.
 3. The integrated circuit structure ofclaim 1 comprising: a plurality of semiconductor strips; and a pluralityof STI regions, with neighboring ones of plurality of STI regionscontacting opposite sidewalls of same ones of the plurality ofsemiconductor strips, wherein the first STI region is one of theplurality of STI regions and contacting sidewalls of two of theplurality of semiconductor strips, and the second STI region contacts asidewall of an outmost one of the plurality of semiconductor strips. 4.The integrated circuit structure of claim 3, wherein the gate dielectriccontacts a top surface and sidewalls of each of the plurality ofsemiconductor strips.
 5. The integrated circuit structure of claim 1,wherein the second dielectric region and the third dielectric regionform a distinguishable interface, and the distinguishable interface hasa continuously and smoothly curving shape.
 6. The integrated circuitstructure of claim 1, wherein the second dielectric region and the thirddielectric region are formed of different materials.
 7. The integratedcircuit structure of claim 1 further comprising: a gate electrode overthe gate dielectric, wherein the gate electrode comprises portionsoverlapping both the first STI region and the second STI region.
 8. Theintegrated circuit structure of claim 1, wherein the first STI regionfurther comprises a first liner dielectric underlying the firstdielectric region, and the second STI region further comprises a secondliner dielectric underlying the second dielectric region.
 9. Theintegrated circuit structure of claim 8, wherein the first linerdielectric and the second liner dielectric are formed of a samedielectric material.
 10. An integrated circuit structure comprising: asemiconductor substrate; a semiconductor strip over a bulk portion ofthe semiconductor substrate; a first Shallow Trench Isolation (STI)region contacting a first sidewall of the semiconductor strip, whereinthe first STI region comprises: a first liner dielectric; a firstdielectric region over the first liner dielectric; and a seconddielectric region over the first dielectric region; and a second STIregion contacting a second sidewall of the semiconductor strip, with thefirst sidewall and the second sidewall being opposite sidewalls of thesemiconductor strip, wherein the second STI region comprises: a secondliner dielectric; and a third dielectric region over and contacting thesecond liner dielectric; and a gate dielectric over and in contact withthe first dielectric region, wherein the gate dielectric is in physicalcontact with the first dielectric region, the second dielectric region,and all top surfaces of the third dielectric region.
 11. The integratedcircuit structure of claim 10, wherein the second dielectric region andthe third dielectric region are formed of a same dielectric material,and top surfaces of the second dielectric region and the thirddielectric region are substantially coplanar with each other.
 12. Theintegrated circuit structure of claim 10, wherein the first linerdielectric and the second liner dielectric are formed of a samedielectric material.
 13. The integrated circuit structure of claim 10,wherein an entirety of a top surface of the first dielectric region iscontinuously and smoothly curved, and wherein the all top surfaces ofthe third dielectric region are coplanar with each other.
 14. Theintegrated circuit structure of claim 10, wherein the first dielectricregion further comprises a planar top surface coplanar with a planar topsurface of the second dielectric region.
 15. The integrated circuitstructure of claim 10, wherein the first STI region has a horizontaldimension greater than a horizontal dimension of the second STI region.16. An integrated circuit structure comprising: a semiconductorsubstrate; a first Shallow Trench Isolation (STI) region extending intothe semiconductor substrate, wherein all dielectric layers in the firstSTI region have a first total count; and a second STI region extendinginto the semiconductor substrate, wherein all dielectric layers in thesecond STI region have a second total count greater than the first totalcount; a semiconductor strip comprising: a first sidewall contacting asidewall of the first STI region; and a second sidewall contacting asidewall of the second STI region; and a gate dielectric on sidewallsand a top surface of a top portion of the semiconductor strip, whereinthe gate dielectric is further in contact with a first top surface ofthe first STI region and a second top surface of the second STI region.17. The integrated circuit structure of claim 16, wherein the second STIregion has one layer more that the first STI region, and each of layersin the first STI region and each of layers in the second STI region arein contact with the gate dielectric.
 18. The integrated circuitstructure of claim 16, wherein the first STI region is narrower than thesecond STI region.
 19. The integrated circuit structure of claim 16further comprising: a gate electrode over the gate dielectric, whereinthe gate electrode comprises portions overlapping both the first STIregion and the second STI region.
 20. The integrated circuit structureof claim 16, wherein all dielectric materials in the first STI regionalso exist in the second STI region, and the second STI region has anadditional layer, with the additional layer formed of a material havinga composition different from all compositions of all dielectric layersin the first STI region.